In higher cells, DNA is complexed with histones and other proteins forming chromatin, and it is at the level of chromatin rather than free DNA that the control of gene activity must operate. Thus, a thorough knowledge of the structure of chromatin is a requirement for understanding gene regulation and development both in the normal and diseased states. Two aspects of chromatin structure will be studied. Firstly, the distribution of mass in the basic chromatin subunit, the nucleosome, will be determined for the normal 'inactive' state and for two variant types (those with acetylated histones, and bound HMG 14 proteins) known to be associated with actively transcribing genes. This approach is made possible by the development of high resolution scanning transmission electron microscopy which allows mass measurements of unstained nucleosomes, and sophisticated computer analysis techniques which allow the 'noisy' images to be aligned and averaged. The results will indicate the types of conformational changes occurring in active chromatin. Secondly, improved electron microscopy preparation techniques have been developed which will be used to study the nucleosomal packing arrangements in thick chromatin fibers. Two key unknown factors in chromatin packing are the linker DNA pathway between one nucleosome and its neighbors, and the contact sites between adjacent nucleosomes in the fiber. The improved techniques allow nucleosome-linker relationships in relaxed chromatin to be characterized, while nucleosome contact sites will be determined following mild crosslinking and fragmentation of the compact fiber.